Digital cameras are widely used which convert images captured by a solid state imaging device, such as CCD, into digital image data and store the data in recording media such as an internal memory and a memory card. The solid state imaging device in such digital cameras includes a semiconductor substrate on which light receiving elements (photodiodes) are arranged in a matrix form, a light shielding layer placed on the semiconductor substrate and having openings (entrances) at the positions corresponding to the light receiving elements, and microlenses located above the light shielding layer. Light rays from an imaging optical system are converged by the microlenses and transmitted through the openings to the light receiving elements.
In these days, the solid state imaging devices are becoming even smaller yet hold more pixels, and the openings are getting smaller in dimension accordingly. As a result, it is becoming difficult to focus light rays toward the light receiving elements strictly by the conventional microlenses. Therefore, solid state imaging devices with an improvement in focusing efficiency are disclosed in Japanese patent No. 2558389, U.S. Pat. No. 6,252,219 (corresponding to Japanese patent laid-open publication No. 11-297974), and U.S. Pat. No. 6,259,083 (corresponding to Japanese patent laid-open publication No. 11-068074).
The solid state imaging devices of the Japanese patent No. 2558389 and the U.S. Pat. No. 6,252,219 both have a concave microlens-forming layer above photoelectric conversion elements (i.e., light receiving elements), and a convex microlens-forming layer above the concave microlens-forming layer. Such a configuration improves the focusing efficiency, compared with the conventional counterparts which only have the convex microlens-forming layer.
In addition, the solid state imaging device of the U.S. Pat. No. 6,252,219 incorporates a concave microlens-forming layer whose concave microlenses have a maximum inclination angle approximately identical to the critical angle for total internal reflection. Thereby, the light rays coming from the convex microlens do not exceed the critical angle for total internal reflection almost anywhere on the concave microlens, and the total internal reflection does not occur in the incident light rays from the convex microlens-forming layer at the concave microlenses. Accordingly, almost all of the incident light rays will pass through the concave microlens-forming layer.
The solid state imaging device of the U.S. Pat. No. 6,259,083 has, at a bottom of a concave lens surface, a well-shaped dug structure which extends toward the light receiving element. This well-shaped dug structure directs incident light rays to the light receiving element.
However, even these solid state imaging devices hardly provide a sufficient efficiency for focusing light rays toward the light receiving element. Especially in these days when the openings are occasionally as small as 1 μm in dimension, it is difficult to focus incident light rays into such small openings.
In addition, while its concave and convex microlens-forming layers can narrow the beams of the vertical incident light to a diameter smaller than the openings, the solid state imaging device of the U.S. Pat. No. 6,252,219 hardly narrow the beams of the oblique incident light to the size of the openings.
Furthermore, in the solid state imaging device of the U.S. Pat. No. 6,259,083, the concave lens surface itself provides little focusing power because a part of it is formed into the well-shaped dug structure. The light rays not focused by the concave lens surface may diffuse when they exit the dug structure, and the focusing efficiency is degraded as a result.